In regard to the deployment of wireless mobile networks in the domestic environment, the design of the antennas is confronted with a particular problem that results from the manner in which the various frequencies are allocated to these networks. Thus, in the case of domestic wireless networks using the IEEE802.11a or Hyperlan2 standard, two distinct frequency blocks, operating in the 5 GHz band, have been allocated to the various service providers as can be seen in the table below.
TABLEAU ATechnologyApplicationFrequency bands (GHz)Europe BRAN/Domestic networks(5.15–5.35)HYPERLAN2 (5.47–5.725)US-IEEE 802.11aDomestic networks(5.15–5.35)(5.725–5.825)
For this reason, in order to cover the two frequency bands, whether it be for a single standard or for two standards simultaneously, various solutions have been proposed.
The most obvious solution consists in using a broadband antenna that covers, at the same time, the two frequency bands defined above. However, this type of antenna covering a broad band of frequencies generally has a complex structure and is expensive. The use of a broadband antenna also has other drawbacks such as the degradation in the performance of the receiver owing to the width of the noise band and to the scrambler capable of operating over the whole band covered by the antenna, this band also comprising the band not allocated to the specific applications in the range 5.35 GHz to 5.47 GHz.
The use of a broadband antenna implies more severe filtering constraints for the transmitter in order to conform to power transmission profiling masks, namely the maximum powers allowed for transmissions both within the allocated band and outside of this band. This leads to additional losses and a higher cost for the equipment.
Furthermore, in wireless networks, at any given time an antenna covers a channel having a bandwidth of around 20 MHz situated in one or the other of the two bands. An alternative solution allowing the drawbacks associated with broadband antennas to be avoided would be to use an antenna whose band of frequencies can be adjusted.
Thus, planar antennas formed, as shown in FIG. 1, by an annular slot 1 are known and which operate at a given frequency f determined by the perimeter of the slot, this slot being supplied by a supply line. More precisely, on a substrate formed by a normal printed circuit metallized on both faces, the annular slot 1, which can be of circular shape or of any other closed shape, is fabricated by etching of the side forming the ground plane of the antenna. The supply line 2 is provided for supplying power to the slot 1, notably by electromagnetic coupling. This is, for example, formed by a line using microstrip technology, positioned on the opposite side of the substrate from the slot 1 and, in the embodiment shown, oriented radially with respect to the circle forming the slot.
The microstrip line—annular slot transition of the antenna is arranged in a known manner such that the slot 1 is located in a short-circuit plane of the line, in other words in a region where the currents are highest. Thus, the supply line after the line-slot transition has a length of around λm/4, where λm is the guided wavelength under the microstrip line. This length can be an odd multiple of λm/4 if the line is terminated by an open circuit, or an even multiple of λm/4 if the line is terminated by a short circuit. Moreover, the diameter p of the slot operating in its fundamental mode is chosen in a known fashion such that p=λf, where λf is the guided wavelength in the slot.
Under these conditions, the distribution of the fields in the slot is as shown in FIG. 2 with two regions of maximum field (CO) and two regions of minimum field (CC). For this reason, it is possible to place a second supply line on the slot at a short-circuit region CC without however degrading the matching at the access on the first supply line while still achieving a good isolation between the two accesses.
Accordingly, the present invention uses this type of structure to obtain a dual-band antenna.